1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus in which a developing roller makes contact with a photosensitive body during an image developing process and separates from the photosensitive body after the developing process is finished.
2. Description of the Related Art
In general, image forming apparatuses, such as laser printers, light emitting diode (LED) printers, digital copiers, facsimiles, and multi-functional devices, transfer an image signal. The image signal is transferred according to an input digital signal, as a visible image, to a printing medium such as a sheet of paper to print the visible image. Such an image forming apparatus generally comprises a developing unit, a laser scanning unit, a transfer medium, and a fixing unit.
The developing unit comprises a photosensitive body on which the visible image is developed. The developing unit also comprises a developing roller to transfer a developer to the photosensitive body and to develop the visible image on the photosensitive medium. An electrostatic latent image corresponding to the visible image is formed on the surface of the photosensitive body by a laser beam irradiated from the laser scanning unit. The developer transferred to the photosensitive body is transferred to the printing medium passing between the photosensitive body and the transfer medium. The printing medium then passes through the fixing unit. As a result, the visible image transferred to the printing medium is fused and fixed onto the printing medium by a fixing unit that applies high temperature and pressure.
Methods of developing an image using the developing unit are classified into a contact type developing method and a non-contact type developing method. In the contact type developing method, the developing roller rotates in contact with the photosensitive body. Alternatively, in the non-contact type developing method, the developing roller does not contact the photosensitive body to rotate and maintains a predetermined developing gap in relation to the photosensitive body.
In the contact type developing method, the developing performance of the developing roller is high. Additionally, a developing structure of the developing roller is simple, and parameters of outer sizes of the developing roller and the photosensitive body are flexible. However, high speed rotation of the developing roller is relatively difficult to achieve. High speed rotation is relatively difficult because the developing roller wears due to contact with the photosensitive body. The developing roller is also sensitive to environmental parameters. Therefore, high resolution may be compromised, and the surface of the developing roller may be pressed when the developing roller is not used for a long period.
In the non-contact type developing method, high resolution can be realized and high speed rotation of the developing roller is possible. However, toner on the surface of the developing roller is often scattered due to the high speed rotation. Thus, the developing roller may become contaminated and may not able to maintain the predetermined developing gap.
Therefore, recently, the developing roller is configured to contact the photosensitive body during a developing process. However, the developing roller does not contact the photosensitive body after the developing process is finished or when the developing unit is not in operation. Consequently, a plurality of gear trains, operatively connected to a drive source for receiving power, are required to operate the developing roller. As a result, the developing roller has a relatively complicated structure.
Accordingly, there is a continual need for an improved image forming apparatus using a combination of contact and non-contact type developing methods to receive power from a drive source.